As the search for Malaysia Airlines Flight 370 continues, attention is turning to the onboard technology that allowed the plane itself, rather than the pilots, to communicate with the ground.

Even if an aircraft's transponder has been disabled, its most basic systems, such as the engines, can also send status information back to ground stations, including the engine manufacturer or the airline. Investigators have now determined that some of these systems were still active on Flight 370 hours after it initially lost contact with air traffic controllers. The question is whether this low-level data is enough to provide new insight on where the plane may have gone.

What is this technology, and how does it work?

Investigators are focusing on data relayed by a system called ACARS, or Aircraft Communications Addressing and Reporting System. In basic versions of the service, the airplane shares data automatically in short radio bursts with airline officials. ACARS allows the plane to send multiple types of messages, including information about fuel levels and engine status. In the case of Air France 447, which plunged into the ocean off the coast of Brazil in 2009, the doomed aircraft sent 29 ACARS transmissions warning of a problem before the plane crashed.

You mentioned basic implementations. There's more than one ACARS?

Think of it like a cable TV package. According to Bill Waldock, an air crash investigator and a professor at Embry-Riddle Aeronautical University, more expensive ACARS packages come with a console that can receive short faxes or send basic messages. Depending on what an airline is willing to pay for, its planes will be able to take advantage of more and better ACARS features.

Okay. So can this technology tell us where Flight 370 is?

Not necessarily — it can really only tell us whether the plane is still functioning.

"Normally, ACARS doesn’t send an actual location," Waldock said. "They're sending essentially system data. They don't indicate altitude or direction. But as long as it’s pinging, you know the airplane is not down."

Investigators now say that based on ACARS information, Flight 370 deviated from its flight path and was in the air for hours after it ceased communications with air traffic controllers.

"The facts are all over the place," said a U.S. official who asked not to be identified because he is not authorized to speak publicly. "It's looking less and less like an accident. It's looking more like a criminal event."

Meanwhile, a satellite operator offered a glimmer of hope Friday when it confirmed it had received "ping" signals from MH370. The company, Inmarsat, said it might be able to use the angle of the incoming transmissions to calculate MH 370's position relative to the satellite. (Here is the news release from the company.)

The following graphic details an arc of possible locations for Flight 370:

ACARS was created in the 1970s as a way to determine automatically what stage of flight a plane was in. Built-in sensors indicated when a plane's doors, parking brake and other equipment was in a given state, which in turn could tell observers whether a plane was at the gate, ready to depart or in the air. In later decades, ACARS became capable of sending detailed performance data to ground teams in real time so that they could be ready to perform maintenance as soon as the craft landed.

If Flight 370 was flying over the ocean, how was ACARS still communicating with the ground?

ACARS messages can be transmitted over more than one protocol. The cheapest and most common way is by sending the data as a packet over radio waves in the VHF range. Here's what a VHF ACARS transmission sounds like, via Wikimedia Commons:

Satellite relays have become another way for these signals to move across the globe, though that's more expensive.

What other satellite technology might help us locate the plane?

Some airlines and aircraft manufacturers, including Boeing — the manufacturer of the missing Malaysia Airlines jet — operate their own satellite subscription services designed to facilitate speedy troubleshooting and maintenance. According to the Associated Press, Malaysia Airlines had not signed up for the program.

Others have pointed to FlightRadar24, a non-commercial flight tracking operation that compiles flight data from the FAA, ADS-B and other sources to show where airplanes are in the world. That information is still subject to limitations, however.

What about the flight's transponder?

Unfortunately, the transponder is part of the mystery: It would be one thing if the plane had crashed at the point where the transponder went dark. In that case we would know where to look for the plane. But that doesn't appear to the case this time. The plane apparently kept flying. So the transponder isn't much help.

Could passenger cellphones hold a clue?

Maybe. But it would depend on a) whether MH370 crossed back over land; b) whether there were cell towers nearby; c) whether the plane was moving low and slow enough to pick up the signals; and d) whether the towers were capable of handing off the devices from one to another well enough to establish a geographic fix as the passengers zipped by at high speeds.

"It's certainly possible under 10,000 feet," said Daniel Berninger, a communications architect and former Bell Labs employee. "But at 35,000 feet, going at 500 mph? I'm not aware of any signals being able to connect. ... The coverage between the cell towers and the handoffs is a whole complex process that doesn't always work. It doesn't even work that great when you're going 55 miles per hour in a car."

Brian Fung covers technology for The Washington Post, focusing on telecom, broadband and digital politics. Before joining the Post, he was the technology correspondent for National Journal and an associate editor at the Atlantic.

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